The present invention pertains to an organic electroluminescent device, and more specifically to a structure for improving the efficiency and lifetime of an organic electroluminescent device.
In recent progress of organic electroluminescent device, the organic EL devices are attractive owing to the merits of high brightness, wide viewing angle, low driving voltage and capability for full color flat portable emissive displays. The normal organic electroluminescent device composes multi-layers of thin film sandwitched between two electrodes. The organic layer composes bole transporting layer, light emission layer and electron transporting layer. Either the electron transporting layer or the hole transporting layer can be designed as the emission layer and the light can be transmitted either way but generally exits through one of the conductive layers. There are many methods to modify one of the conductive layers for the emission of light there through but it has been observed that the most efficient organic electroluminescent device includes one conductive layer, which is transparent to the light being emitted. The widely used material for this conductive and transparent layer is indium-tin-oxide (ITO), which is usually deposited on a transparent substrate, for example, the glass plate.
Although the multi-layer structure is normally used in constructing the organic electroluminescent devices, the interface between the electron transporting layer and hole transporting layer is not compatible and results in a bad junction in the interface of the different layers and eventually causes the stripped off phenomenon in the device under high temperature condition. Also the lifetime of multi-layered organic electroluminescent devices is influenced by the abrupt change of interface interface between the hole and electron transporting layers when the organic EL device is under voltage bias.
Significant improvement of electroluminescent device has been achieved in the prior art (See U.S. Pat. No. 5,925,980). In the patent, a structure of electroluminescent device which comprises a hole transporting region, an electron transporting region and a graduated region disseminated between the hole transporting region and the electron transporting region is provided. The graduated region changes, either in steps or continuously, from the hole transporting organic material adjacent to the hole transporting region to the electron transporting organic material adjacent to the electron transporting region. Further improvement is still needed for applications where lifetime is a primary concern. The lifetime of an organic electroluminescent device is affected by the stability of both the bulk morphology of the hole transporting materials and the interface between the hole and electron transporting layers when the organic electroluminescent device is under bias.
Several schemes have also been proposed to address the problem of bulk morphology stability of the hole transporting material in an organic electroluminescent device. Among other things, the lifetime of the electroluminescent device is improved by the elimination of heterojunction in a continuous organic medium (See U.S. Pat. No. 6,130,001). In the patent, it provides an organic electroluminescent layer which comprises a continuous organic medium AxBy where A and B are components capable of transporting electrons and holes, respectively, where x represents the content of A component with a value ranging from 0 adjacent to the anode to 100% adjacent to the cathode, and y represents the content of B component with a value ranging from 0 adjacent to the cathode to 100% adjacent to the anode. The lifetime of the device is hence improved by the elimination of heterojunctions in the continuous organic medium.
Generally, there is always a need to provide a smooth reliable region so that the interface effect can be reduced to a minimum. However, the smooth reliable region should not come at the expense of the efficiency of the organic electroluminescent device. Also, the elimination of the difference in the interface between different layers will give the electroluminescent device with greatly improved reliability and lifetime.
It is a purpose of the present invention to provide a new and simplified organic electroluminescent device with an improved lifetime.
It is another purpose of the present invention to provide a new and simplified organic electroluminescent device to enhance electron injection capability and to achieve emitting high efficiency.
It is still another purpose of the present invention to provide a new and simplified organic electroluminescent device with improved stability.
The above problems and others are at least partially solved and the above purposes and others are realized in an organic electroluminescent device including a single organic coating layer with a continuous medium inserted between the anode electrode and the cathode electrode.
The single organic layer in this invention comprises three components, which include hole transporting material, electron transporting material, and electron injecting material. The composition medium for forming the single organic layer is AxByCz, wherein A is the component capable of injecting and transporting holes, B is the component capable of transporting electrons, C is the component capable of injecting electrons and x, y, and z denote the content of component A, B, and C respectively. The single organic layer with different materials is a mixed and continuous organic medium without heterojunctions.
Furthermore, the content x, y, and z in component A, B, and C have the characteristics that the sum of x, y, and z is 100%. Moreover, x has a maximum value adjacent to the anode (may include a hole injection layer), and the sum of y and z has a maximum value adjacent to the cathode. It should be pointed out that the organic medium for use further comprises at least one fluorescent, phosphorescent dye or pigment in the single organic electroluminescent layer of the formula AxByCz.
The component C is an electron injection component including an organic medium with high electron affinity. On the other hand, the component C may also comprise inorganic material such as a low work function metal, metal alloy, or metal compound.